U.S. Pat. No. 3,587,775 discloses a seismic acoustic source that generates sound by electrolytically dissociating water into gaseous hydrogen and oxygen. The hydrogen and oxygen are then allowed to explosively recombine. The explosive recombination results in the sound waves. The source has been built in a tubular housing hung on a wireline cable within a well borehole.
A portion of the outer wall of the housing is made from a flexible material, thus defining an annular compartment. Surface equipment provides both dissociating power and the firing power. The ends of the portion are clamped firmly to the outer wall of the housing whereas longitudinal slots in the tubular housing wall permit waves generated to travel outwardly through the borehole fluid and into the formation.
In such a way a relatively compact device is designed. The apparatus is operable and firable repetitively at various levels in a borehole without requiring the addition of any material from the surface, although many problems remain.
For example, although firing, the dissociated hydrogen and oxygen will result in inflating the flexible wall portion throughout the slots. Thus the source cannot be considered pointlike. Moreover waves will propagate in all directions thus dividing power both in the direction of the borehole wall, and upwardly and downwardly into the borehole fluid.
It is also a disadvantage that the device can be employed only in a separate logging procedure instead of in combination with drilling operations.
U.S. Pat. No. 2,679,205 discloses seismic sound source generating waves similar to that disclosed in U.S. Pat. No. 2,679,775. In particular U.S. Pat. No. 2,679,205 discloses a sound source comprising a compartment for a gaseous hydrogen-oxygen mixture which is flexible as a whole. Although a more pointlike source device results, unpredictable sound direction components and disadvantageous power dissipation remain.
U.S. Pat. No. 4,207,619 discloses another seismic well logging system and method. In particular, it discloses a source device arranged at the lower end of the drill bit string and capable of producing a sonic pulse. The pulses may be produced, for example, by induced vibrations, implosions, explosions, or sudden injections of fluid against the bottom of the borehole. The source device is driven and controlled, for example, by a sequence timer whereas pulses are generated whenever the drilling operation is temporarily halted to add a new segment of drill pipe to the drill string. Thus measuring is carried out while drilling in a semi-continuous way. As a further feature pulses at a single level may be summed to enhance the distinction between the pulses and the noise generated by the engines associated with the drilling operations. The above logging method uses surface detector systems to detect the waves generated which have passed the subsurface formation, thereby having been refracted and/or reflected, resulting in formation mappings and drill steering information.
Conditions for arranging and driving a source device near the bit are very severe. High rotation speeds, severe drilling and grinding conditions, and noise will occur as a result of the drill bit hammering onto the borehole bottom.
Although an overall method and related device for transmitting, receiving, and interpreting acoustical signal data from start to finish are presented in the above document, difficulties met in arranging such a device near the drill bit are not addressed.
Thus, it is an object of the invention to present a method and a logging device enabling a substantially undisturbed and immediate transfer of acoustic power from the device into the subsurface formation.
It is a further object of the invention to provide such a method and device of creating acoustic pulses at any moment and at any borehole level desired as long as the drill operations continue.
It is another object of the invention to provide a method and a logging device to create pulses from near or on the drill bit which are clearly distinguishable from further noise, such as bit noise.
It is yet a further object of the invention to drive and control the logging device continuously and reliably.